1. Field of the Invention
The present invention relates to a powdery dispersant for hydraulic compounds such as cement, gypsum etc., particularly for use in grout mortar and to a hydraulic composition comprising the same.
2. Description of the Prior Art
As a dispersant for hydraulic compounds, a monovalent metal salt of a polycarboxylic acid-based copolymer has been used usually as a liquid product because of its high dispersibility. For further improvement of convenience, the metal salt is formed into a powdery dispersant, and a product comprising this powdery dispersant premixed with powdery materials such as hydraulic compounds, aggregate etc. has been developed.
However, there are cases where the monovalent metal salt of a polycarboxylic acid-based copolymer is hardly dry-powdered at room temperature, or even if it can be dry-powdered, the long-term shelf stability thereof prior to use, that is, the absence of adhesion (pressure resistance) attributable to the pressure in a package and the absence of adhesion and deterioration in dispersibility (humidity resistance) attributable to moisture absorption, is not satisfied in high levels.
As the method of obtaining a powdery dispersant, JP-B 2669761 (JP-A 6-239652) discloses a method of spray-drying a liquid polycarboxylic acid-based copolymer and inorganic powder, JP-A 9-309756 discloses a method of converting a copolymer consisting of a polyalkylene glycol alkenyl ether having a few alkylene oxide molecules added thereto and maleic anhydride into a polyvalent metal salt, JP-A 11-310444 discloses a method of using a water-soluble vinyl copolymer in combination with a water-sparingly-soluble, specific metal salt or metal hydroxide, and JP-A 2000-26145 discloses a method of dry-powdering a (meth)acrylate-based cement dispersant having a specific structure wherein the average number of alkylene oxide molecules added is 20 to 109.
When a powdery dispersant is used in a large amount in large scale in a factory in an environment where the temperature and humidity are not necessarily regulated, there is demand for a powdery dispersant with further improvements in pressure resistance and humidity resistance. On the other hand, there is also demand for a powdery dispersant with dispersion performance which is particularly capable of dispersing mortar in a hydraulic composition without conferring excessive viscosity on the mortar, particularly for use in a large amount of powder (particularly for use in grout mortar) which should be endowed with suitable viscosity and dispersed on the spot. In the prior techniques, however, a means of mixing an inorganic powder not contributing to dispersion of hydraulic compounds or a means of incorporating a large amount of salts is adopted, so there are worries about reduction in the efficiency of dispersion by the powdery dispersant or the deterioration of physical properties which may be caused by the powdery dispersant after hardening of the dispersant system.
The powdery dispersant is required to have the long-term shelf stability thereof prior to use, that is, the absence of adhesion (pressure resistance) attributable to the pressure in a package and the absence of adhesion and deterioration in dispersibility (humidity resistance) attributable to moisture absorption, but the conventional powdery dispersants, particularly comprising a monovalent metal salt of polycarboxylic acid-based copolymer, are still poor in pressure resistance and humidity resistance. Further, when dry powdering is conducted industrially, it is important that the viscosity of an aqueous solution of starting materials is not increased to such an extent as not to hinder pumping, etc., and further when the powdery dispersant is used in grout mortar, the resulting hydraulic composition just after kneading is required to have high flowability and suitable separation resistance, but the conventional dispersants are still not satisfactory for these requirements.